Sodium-ion batteries have emerged as a promising alternative to lithium-ion systems due to the abundance and lower cost of sodium. A critical component influencing their performance is the electrolyte, which facilitates ion transport between electrodes and plays a key role in forming the solid-electrolyte interphase (SEI). Electrolytes in sodium-ion batteries can be categorized into liquid, solid, and hybrid systems, each with distinct chemistries and implications for conductivity, stability, and safety.

Liquid electrolytes remain the most widely used due to their high ionic conductivity and effective electrode wetting. These typically consist of sodium salts dissolved in organic solvents. Common sodium salts include sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), and sodium bis(fluorosulfonyl)imide (NaFSI). The choice of salt affects ionic dissociation and electrochemical stability. NaPF6 is frequently used due to its relatively high conductivity and stability, though it is sensitive to moisture. NaClO4 offers cost advantages but presents safety concerns due to its oxidizing nature. NaFSI has gained attention for its ability to form stable SEI layers, improving cycle life.

Solvent selection is equally critical. Carbonate-based solvents such as ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) are common due to their wide electrochemical stability window and good solvation properties. EC is particularly effective in promoting stable SEI formation but has a high melting point, limiting low-temperature performance. Blending EC with linear carbonates like DMC or ethyl methyl carbonate (EMC) improves low-temperature behavior while maintaining SEI quality. Ether-based solvents, such as diglyme, have also been explored for their ability to enhance sodium-ion mobility, though they often exhibit narrower stability windows compared to carbonates.

Additives play a crucial role in modifying electrolyte properties. Fluoroethylene carbonate (FEC) is widely used to improve SEI stability by promoting the formation of inorganic components like NaF, which enhances interfacial resistance to degradation. Vinylene carbonate (VC) is another additive that aids in forming a robust SEI but may increase electrolyte viscosity. Boron-based additives, such as lithium bis(oxalato)borate (LiBOB) analogs, have been adapted for sodium systems to improve thermal stability and reduce gas generation during cycling.

Solid electrolytes offer inherent safety advantages by eliminating flammable organic solvents. They are categorized into inorganic solid electrolytes (ISEs) and solid polymer electrolytes (SPEs). ISEs, such as sodium superionic conductors (NASICON-type Na3Zr2Si2PO12 and β-alumina), exhibit high ionic conductivity at room temperature but often suffer from brittleness and poor interfacial contact with electrodes. Sulfide-based solid electrolytes, like Na3PS4, show higher ductility and better interfacial properties but are sensitive to moisture, releasing toxic hydrogen sulfide.

SPEs, typically composed of sodium salts dissolved in polymer matrices like polyethylene oxide (PEO), provide flexibility and easier processing. However, their low ionic conductivity at room temperature remains a challenge. Incorporating ceramic fillers, such as Al2O3 or TiO2, into SPEs can enhance conductivity by disrupting polymer crystallinity and providing additional ion transport pathways. Cross-linking polymers or designing block copolymers are other strategies to improve mechanical strength without sacrificing ionic mobility.

Hybrid electrolytes combine liquid and solid components to balance performance and safety. These systems often involve a porous solid matrix soaked in a liquid electrolyte, offering higher conductivity than pure solid electrolytes while reducing leakage risks. Gel polymer electrolytes (GPEs) are a common hybrid type, where a liquid electrolyte is immobilized within a polymer network. The polymer provides structural integrity, while the liquid component maintains high ionic conductivity. Another approach is using quasi-solid-state electrolytes with ionic liquids, which offer non-flammability and wide electrochemical windows.

Ionic conductivity is a primary performance metric for electrolytes. Liquid electrolytes typically achieve conductivities in the range of 10–20 mS/cm at room temperature, while solid-state systems range from 0.1–10 mS/cm depending on composition and microstructure. Hybrid systems often fall between these ranges, with GPEs reaching 1–5 mS/cm. Achieving high conductivity in solid and hybrid systems requires optimizing salt concentration, polymer chemistry, and filler distribution to minimize ion transport barriers.

SEI formation is another critical factor influenced by electrolyte composition. A stable SEI prevents continuous electrolyte decomposition and sodium dendrite growth, which can lead to short circuits. In liquid electrolytes, the reduction of solvents and salts forms the SEI, with additives like FEC promoting a more uniform and ionically conductive layer. In solid-state systems, the SEI is less pronounced, but interfacial resistance remains a challenge. Surface treatments or introducing artificial SEI layers can mitigate this issue.

Safety considerations are paramount, especially for large-scale applications. Liquid electrolytes pose flammability risks, particularly when using volatile carbonates. Solid electrolytes eliminate this hazard but may introduce mechanical stability concerns. Thermal stability is another key factor; sodium salts like NaFSI exhibit better thermal resilience than NaPF6, which decomposes at elevated temperatures. Hybrid systems must balance non-flammability with long-term stability against phase separation or solvent evaporation.

Stability against sodium metal is crucial for high-energy-density applications. While liquid electrolytes often react with sodium metal, forming an SEI, repeated cycling can lead to dendrite penetration. Solid electrolytes can physically block dendrite growth but require intimate electrode contact to prevent interfacial resistance buildup. Hybrid systems may offer a compromise, though long-term cycling data remains limited.

In summary, electrolyte design in sodium-ion batteries involves trade-offs between ionic conductivity, interfacial stability, and safety. Liquid electrolytes provide high performance but require careful additive engineering to enhance safety. Solid electrolytes offer inherent safety benefits but face challenges in conductivity and interfacial engineering. Hybrid systems aim to bridge this gap but require further development to achieve widespread adoption. Advances in salt formulations, solvent blends, and additive chemistry will continue to play a pivotal role in optimizing sodium-ion battery performance for diverse applications.